A Planet ‘Just Right’ for Life? Perhaps, if It Exists

It was big news back in September 2010 when a group led by Steven S. Vogt of the University of California, Santa Cruz, and R. Paul Butler of the Carnegie Institution for Science said they had discovered a small planet circling a small red star in the constellation Libra, at a distance smack in the middle of the so-called Goldilocks zone — that “just right” region where water on the surface is possible.

If confirmed, Gliese 581g, as it is known, would be the first known exoplanet — that is, a planet outside our own solar system — that could support life. But within weeks, a group of exoplanet astronomers based at the Geneva Observatory in Switzerland under the leadership of Michel Mayor said they could not find the planet in their own extensive data on the star. After other papers appeared questioning the statistical significance of the new planet, most astronomers consigned it to the bin of failed dreams.

Typical was the response from Artie P. Hatzes, a former student of Dr. Vogt’s who is now at the Thuringian State Observatory in Tautenburg, Germany, who said it pained him to see his old mentor sticking to a conclusion “that is obviously wrong.”

Dr. Hatzes called Gliese 581g “a marginal detection” that was not supported by additional data, something that happens often. “I think that no amount of fancy analysis can replace having high-quality data and more of it,” he said.

The impasse has led some astronomers to suggest that an outsider look at the data, but everyone is too busy with his or her own research. In the two years since Gliese 581g (pronounced GLEE-za) was announced, NASA’s Kepler satellite has identified more than 2,300 candidate planets in a small portion of the Milky Way in the Cygnus constellation, which scientists are sifting in hopes of soon discovering what some call Earth 2.0.

If nothing else, the dispute illustrates just how hard it is to find out what is going on only a short distance away in the universe. The star Gliese 581 is about 20 light-years away, next door in cosmic terms. The evidence for its planets comes from slight periodic variations in the star’s velocity, caused by gravitational tugs of planets swirling around it. The heavier the planet and the closer it is to its star, the bigger the tug, so it can take years to build up and parse the data for an entire system — the so-called wobble method.

It was on this basis, using sensitive spectrography that goes by the acronym Harps on a telescope in Chile, that the Geneva team concluded by 2007 that there were four planets circling the star, two of them on either side of the “habitable zone.”

Three years later, Dr. Vogt and his colleagues added two more planets based on their own observations with the Keck telescope in Hawaii, including the one in the middle of the habitable zone, circling the star every 37 days.

The European astronomers were quick to say the new planet was not in their own data. But much to the annoyance of Dr. Vogt and his colleagues, that data was not made public for a year, in a paper posted to the Internet by Thierry Forveille of the Grenoble Observatory in France, the lead author.

Dr. Forveille and his colleagues concluded that their data were best fit by a model in which Gliese 581 had four planets, one of which had an elongated elliptical orbit of 69 days. Such a planet could masquerade as one with half that orbital period, they said, perhaps explaining Dr. Vogt’s planet.

Dr. Vogt and his colleagues contend that such an arrangement would be unstable, leading to colliding planets within 200,000 years. Redoing the analysis and assuming that the orbits were all circular, they found leftover data points that could be explained by a fifth planet with an orbit of about 32 days, snugly in the habitable zone, and a mass slightly more than twice that of Earth, essentially confirming their original Goldilocks claim.

Getting this new study published was a tricky problem. Because the Forveille data had not yet been published in a peer-reviewed journal, Dr. Vogt was not able to publish his analysis of that data in The Astrophysical Journal, the journal of choice for American astronomers.

So they sent it to Astronomische Nachrichten, one of the oldest astronomy journals in the world, but rarely the venue today for groundbreaking news. “So we will now publish their own data for them for all the world to see and analyze, and then judge for themselves whether the Swiss’s rebuttal claims have any credibility,” Dr. Vogt wrote.

In an e-mail, Dr. Forveille shrugged off the new report, writing that his own team’s computer simulations showed that the planetary system with elliptical orbits was stable at least over 900,000 years.

More to the point, he and other astronomers said, was that Dr. Vogt’s planet had a 4 percent chance of being a false alarm. That is far above the 1 percent chance normally considered a benchmark for planet detection, and much bigger than the margin Dr. Vogt and his colleagues had cited in their first paper.

“Increasing false alarm probabilities with more data is of course never a good sign,” Dr. Forveille said. Greg Laughlin of the University of California, Santa Cruz, an expert on planetary dynamics, said the chances of a false alarm rise to 12 percent when uncertainties about the star Gliese 581 itself — stellar noise — are added in.

But, he added, Dr. Vogt still had “a decent shot”of being right. It comes down to whether four planets with eccentric orbits or five planets with circular ones are a more economical fit to the data. Assuming that the planets’ orbits all lie in the same plane, Dr. Laughlin explained, tips the balance toward the extra planet. But, of course, more data is needed. He said, “I thus believe that it is likely that if the peak holds up as more data comes in, the planetary explanation is likely to be correct.”

Eric Ford, an astronomer at the University of Florida who is involved with Kepler, noted that Carl Sagan had maintained that extraordinary claims require extraordinary evidence.

“In my judgment,” Dr. Ford said, “we do not have extraordinary evidence for the claimed planet Gliese 581g, at least not yet.”

Dr. Hatzes agreed, but added, “I am sure this will not be the last we have heard of GL 581g.”

A version of this article appears in print on August 21, 2012, on page D2 of the New York edition with the headline: A Planet ‘Just Right’ for Life? Perhaps, if It Exists. Order Reprints|Today's Paper|Subscribe